Camp Six is composed of windowless, steel cells where inmates are confined for at least 22 hours a day.

The US authorities say the new facility allows inmates more "privacy" but Amnesty says Camp Six has created increased conditions of extreme isolation, to the detriment of prisoners' mental health.

Amnesty says Camp Six is more "enclosed" than Camp Five, where detainees are thought to have been held in confinement for up to 24 hours at a time.

"While conditions in both camps are extremely harsh, according to a contact who has viewed cells in each facility, the difference in Camp Six is that detainees have no way of knowing whether it is day or night," the report says.

"One detainee has described Camp Six as being a 'dungeon above the ground'."

Findings 'unsurprising'

Moazzam Begg, a former detainee who was released without charge from GuantanamoBay in January 2005, told Al Jazeera that he was unsurprised at the findings contained in the Amnesty report.

"I think the facts that are now coming out are highly unsurprising considering that the US government has always maintained [the] posture that people need to be broken – physically, spiritually and mentally – in order that they become more compliant," he said.

Begg said the US's lack of communication to detainees increased their mental distress further.

"[Detainees] don’t have meaningful communication with their families. They don't know when or if they will ever face any trial or charge," he said.

"To be in the situation where one can't walk more than three steps in any direction, because the cell one is in is only eight feet by six feet, is one of the most harrowing experiences one can have."